To get an idea of how an embryo develops, we’ll look at the development of a frog.

We’ll use the frog’s developing embryo to learn some terms for understanding the orientation of the embryo. This embryo is starting to take on the shape of the tadpole, with the head and tail starting to develop.

The front or head end of the embryo is called the anterior.

The rear end is the posterior.

The top or back of the embryo is called the dorsal side and the bottom is the ventral side.

These terms are rather like a biological version of the points of the compass.

See if you can identify the anterior, posterior, ventral and dorsal regions of this adult frog.

Drag the labels into place, and click Submit to see if you’re correct.

Correct: That’s right!

All others: Sorry, that’s not quite right.

All: Remember that the anterior is going to be the head end of the embryo.

The posterior is the tail, and dorsal and ventral refer to the back and belly of the embryo respectively.

Now that we have our ‘biological compass’ to describe the orientation of an embryo, let’s go back to our frog egg. We can think of the fertilized egg as a planet.

Instead of the north and south hemispheres, the egg is divided into animal and vegetal hemispheres.

The word ‘animal’ in this context is equivalent to ‘dorsal’, and ‘vegetal’ is equivalent to ventral.

The animal pole is at the most dorsal point of the egg and the vegetal pole is at the most ventral point.

The cytoplasm in the egg is not uniformly distributed.

The cytoplasm toward the vegetal pole is denser because it's filled with nutrients.

These nutrients will eventually become the embryo's food source—the yolk.

The embryo undergoes a series of mitotic cell divisions. This sequence of cell divisions is called cleavage.

Cleavage takes place with little cell growth, so that the 2, 4, 8, 16, and even 32-cell embryo is around the same size as the single-celled egg.

The nutrient-rich cells near the vegetal pole divide more slowly and are therefore larger than the less dense cells in the animal hemisphere.

As cleavage proceeds, the dividing ball of cells gradually forms a cavity in its center. The embryo is called the blastula at this stage.

If we cut the embryo in half at this stage, we can see the cavity in the center of the developing mass of cells. The cavity is called the blastocoel.

We can also see that the cells in the animal hemisphere are smaller than those in the vegetal hemisphere.

Some of the vegetal cells also start to process nutrients to make yolk, which will be used as the embryo develops.

How does the ball of cells start to change to a more complex structure?

Think of pushing in on a hollow rubber ball with your thumbs. This type of inward movement is called invagination.

This is essentially what happens during gastrulation, the process by which cells of the blastula invaginate to form the next stage of embryonic development, the gastrula.

The gastrulation process creates what will eventually be the animal’s intestinal tract.

Invagination is an example of the movement of a sheet of cells during development to drive morphogenesis.

By the time gastrulation is complete, the three primary cell types or germ layers are in place.

These layers are called the ectoderm, mesoderm and endoderm. Ectoderm produces the outer layer of skin and the nervous tissues, mesoderm produces muscle, heart and kidney and endoderm produces digestive organs.

These layers will eventually give rise to the adult organs in a process called organogenesis.

Organogenesis is first seen in the developing embryo as the formation of a tube along the dorsal midline.

The tube is made up of cells rolling up along the anterior-posterior axis of the embryo.

This tube will grow structures of the animal’s nervous system, the brain and spinal cord.

Cell shape changes, differential adhesion between cells and cell movements are all important in these kinds of morphogenetic processes.

Shape changes are fueled by the cell's cytoskeleton, and differential adhesion between neighboring cells is aided by special adhesion molecules on the cell's surface.

Cells can also move over one another by extending finger-like projections which help a cell drag itself forwards.

Amphibians like frogs lay their eggs in water.

But reptiles and birds live mainly on land, and their eggs have to be protected from drying out. Most mammals don’t lay eggs at all, but their eggs also need to be protected.

Several protective embryonic membranes have evolved, and they are seen in reptiles, birds and mammals.

In this bird embryo, we can see the membranes: the amnion, chorion, allantois, and yolk sac.

The amnion encloses the embryo in a fluid-filled sac which prevents the animal from drying out.

The development of the amnion is an important evolutionary adaptation because it enabled animals to live on land. You may recall from earlier in the course that reptiles, birds, and mammals are sometimes called amniotes.

The chorion provides shock-absorbing protection.

The allantois is essentially a receptacle for waste products

The yolk sac encloses the growing embryo’s food supply. The yolk sac is important in birds and reptiles, where the egg develops independently of the parent.

By contrast, in mammals, the yolk sac plays a smaller role because the embryo gets most of its nutrients from the mother.

Now that we’ve explored the formation of the embryonic membranes, see if you can match the membranes with their correct function.

Click Submit to see if you're correct.

Correct: That’s right

All others: Sorry, that’s not quite right.

All: The amnion prevents the embryo from drying out.

The chorion provides shock- absorbing protection.

The allantois holds the embryo’s waste products.

The yolk sac holds the food supply for the developing embryo.

Now that we’ve studied the structure of the developing embryo, let's see how scientists learned about how development is controlled.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education